Platelet activation is an ordered sequence of events that begins with the binding of an agonist to its receptor on the platelet surface and concludes with platelet aggregation and secretion. Ca2+ ions play a critical role in these events. Extracellular Ca2+ is necessary for some of the events that take place on the platelet surface, such as fibrinogen binding and, ultimately, platelet aggregation. Intracellular Ca2+ serves as a secondary messenger during platelet activation. The studies that are contained in this proposal focus upon two main areas: the mechanisms of Ca2+ transport that help to maintain Ca2+ homeostasis in platelets and the properties of the Ca2+ binding proteins on the platelet surface, especially glycoproteins IIb and IIIa. Some of the specific issues that will be addressed are: [1]\Identification of the mechanism that mediates Ca2+ efflux across the platelet plasma membrane. [2] Consideration of the role of plasma membrane glycoproteins IIb and IIIa in Ca2+ transport. [3]\Re-examination of the relative contributions of the platelet dense tubular system and mitochondria to Ca2+ homeostasis in resting and stimulated platelets. [4]\Examination of the mechanisms involved in platelet recovery from reversible activation, including the possibility that Na+/Ca2+ exchange is involved. [5] Elucidation of the changes in the IIb/IIIa complex that occur when platelets are activated and the complex is altered to expose the fibrinogen receptor. [6] Identification of Ca2+ binding sites on the cytosolic surface of the platelet plasma membrane that may play a role in platelet activation. [7] Identification of additional Ca2+ binding proteins on the platelet surface.